1. Field of the Invention
The present invention relates to an air-fuel control apparatus for a fuel injection type internal combustion engine for an automobile.
2. Discussion of Background
It is necessary to maintain an air-fuel ratio for exhaust gas (hereinbelow, referred to simply as an air-fuel ratio) around a theoretical air-fuel ratio as possible in an internal combustion engine, particularly, one provided with a three-way catalytic exhaust gas purifying means. For this purpose, there has been practically used, for instance, an air-fuel ratio control apparatus which comprises an oxygen concentration sensor for detecting an air-fuel ratio on the basis of the concentration of oxygen contained in exhaust gas, an air-fuel ratio control means having an electronically-controlled fuel injection device for controlling the air-fuel ratio of a gas mixture to be supplied to the burning chamber of the engine by controlling the quantity of fuel to be injected, and an electronic control device for effecting feed-back control of a fuel injection quantity by the electronically controlled fuel injection device so that the value of the air-fuel ratio approaches the value of a theoretical air-fuel ratio in response to a calculated air-fuel ratio obtained by an output of oxygen concentration.
In the conventional air-fuel ratio control apparatus, the feed-back control was carried out so that the air-fuel ratio apporaches the theoretical air-fuel ratio, whereby exhaust gas purifying effect could be sufficiently improved by using the three catalytic system disposed in an exhaust gas discharging unit. However, although the conventional air-fuel ratio control apparatus could improve an exhaust gas purifying function, it is difficult to maintain a theoretical air-fuel ratio even under such condition that the engine is operated in a practically allowable state and the air-fuel ratio is at a lean side, and accordingly the performance of the engine can not sufficiently be obtained. Further, the feed-back control could not be obtained when a high torque is required by rendering the air-fuel ratio to be rich in a state of the full open of the engine. Accordingly, a precise correction of the air fuel ratio could not be obtained when an air-fuel ratio varies in a rich region because of variation with time and the scattering in dimensional value of the parts.
In particular, the above-mentioned problem becomes serious in an internal combustion engine with a supercharger. When a predetermined air-fuel ratio in the rich region is deviated toward further rich side, it falls in an inflamable range to thereby cause firing. On the contrary, when the predetermined air-fuel ratio is shifted into the lean side, the temperature of exhaust gas becomes high to thereby cause damage of the parts of the internal combustion engine.
In order to eliminate the above-mentioned problem, there has been a proposal such that an air-fuel ratio for an internal combustion engine is feed-back-controlled to give a predetermined air-fuel ratio by using a sensor which continuously measures an air-fuel ratio in a region covering the lean side and the rich side on the basis of specified components contained in exhaust gas (such sensor is referred to as an air-fuel ratio sensor hereinbelow).
As an example of the air-fuel ratio control in the conventional apparatus, a negative pressure in an intake air tube and the number of revolution of the engine are detected, and the fuel supplying device is controlled to obtain a target air-fuel ratio on the basis of the detected values of the pressure and the revolution number. In such control system, sufficient warming-up of the engine at the time immediately after starting the engine could not be obtained. Thus, the conventional air-fuel ratio control apparatus had such disadvantages that an effective feed-back control could not be obtained in a region that the engine does not operate stably if a relatively high torque is outputted by shifting an air-fuel ratio toward the rich side. Therefore, in the conventional apparatus, high, fine control of air-fuel ratio can not be obtained in the entire operable region of engine.